The production of arylene sulfide polymers by the reaction of at least one polyhalo-substituted cyclic compound with an alkali metal sulfide in a reaction medium comprising a polar organic compound is disclosed in U.S. Pat. No. 3,354,129, issued to Edmonds and Hill. The disclosure of this Edmonds and Hill patent is herein incorporated by reference.
The poly(arylene) sulfide, hereinafter PAS, of most industrial importance is a semi-crystalline poly(phenylene) sulfide polymer, hereinafter PPS, which is produced commercially from a sodium sulfide equivalent, e.g., an approximately equal mixture of sodium hydrosulfide (NaSH) and sodium hydroxide (NaOH) followed by the addition of p-dichlorobenzene (DCB), in a reaction medium of a cyclic amide, such as N-methyl-2-pyrrolidone (NMP).
It is well known that properties and also processing characteristics of PAS polymer, such as PPS, can be improved by treating the polymers obtained directly from the reactor with a variety of fluid treating agents, which can be selected from the group including metal salts derived from the elements of Periodic Groups IA, II and IIIA, organic acids, and inorganic basic compounds. Specific treating agents include lithium acetate, sodium carbonate, potassium bicaronate, sodium bicarbonate, barium acetate, calcium acetate, sodium acetate, magnesium propionate, zinc nitrate, aluminum acetate, glacial acetic acid, concentrated ammonium hydroxide (30%), sodium hydroxide and the like. A particularly preferred compound because of special efficacy in the process is sodium acetate. The thus modified polymer is known to exhibit improved physical properties, as well as improved processing characteristics.
Further, it is known that impurities present in PAS polymers can be chemically treated in a manner such that the PAS polymers are essentially freed of ash forming material, which includes ions from the salts (e.g., Na.sup.+ ions), and concurrently cured to a higher molecular weight product exhibiting reduced melt flow. The chemical treating agents for this further treatment are water soluble and can be selected from hydrogen peroxide, an alkali metal or alkaline earth metal hypochlorides, sulfur acids, sulfonyl halides, chlorine, nitrogen dioxide, chromium trioxide, alkali metal permanganates, acetic acid and nitric acid. A particularly preferred compound because of its special efficiency is acetic acid.
The acetic acid treatment for purifying a modified or unmodified PPS resin, which is essentially a washing operation, includes contacting a resin with an aqueous acidic solution to form a slurry, wherein a pH in a range of about 5 to about 6 is desired for the slurry. The acidic solution, which is contacted with PPS polymer for washing, needs to have a pH and acid content sufficient to extract the salt or ash ions from the polymer. For the acidic solution washing, the PPS resin is preferably slurried with condensed or deionized water and acetic acid in an amount of about 0.05 volume-%, (based on the volume of the polymer slurry) is added to obtain the desired pH for the slurry. Accordingly, the acid washing operation is carried out in a batch fashion, so that the proper amount of acid can be added to a fixed volume of polymer slurry. In the past, this acetic acid has been manually added to a wash vessel containing the polymer slurry.
The amount of acid added to the polymer slurry is critical, since too little acid fails to remove the impurities ions and too much acid adversely affects many desirable properties of the resin.
Accordingly, it is an object of this invention to improve control of the amount of acid added to a polymer slurry for removing impurity ions.
It is a further object of this invention to increase resin throughput in an acid washing operation.
It is a still further object of this invention to provide a continuous acid treatment washing operation for purifying a polymer resin.
Other objects and aspects, as well as the advantages of this invention will be apparent to those skilled in the art upon reading the specification and the appended claims.